Was wondering... about $1000 for SLI 580, about $730 for SLI 570.
Is the performance hit of the 570s instead of those 580s really worth that ~$250?
If its more than 20% or so, I'll consider it. But can anyone tell me if its that much of a difference?

As supa mentioned, it really depends on your resolution. If you're gaming above 1080p then the performance difference may be noticable/significant,however if you're at 1080p or lower there is no real point in the minor performance increase.

I agree, you would be best going to a single 580, and if you want to sli later you can pick up another. The real performance difference of a 570 vs a 580 may not be visually noticeable that much in most games, BF3 "could" be the exception to the rule but I can't speak from experience there.